1. Field of the Invention
The present invention generally relates to medical devices, and more particularly, to a technique for regulating the output level of a magnetic field transmitter for controlling implantable devices.
2. Description of the Prior Art
As implantable medical devices in general and cardiac pacers in particular became more complex in operation, the desirability for non-invasive transfer of data between the implant and an external device could be seen. In cardiac pacing, transfer of signals from an external device to the implant to modify operating parameters is called programming. Data is also transferred from the cardiac pacer to the external device to provide various monitoring information. These transfers are often termed telemetry.
U.S. Pat. Nos. 4,142,533 and 4,332,256 both issued to Brownlee et al., describe one approach to data communication between an implanted cardiac pacer and an external device. Though the primary feature discussed is telemetry of monitoring data from the implant, the importance of a non-invasive approach is emphasized.
A specifically two-way transmission system is shown in U.S. Pat. No. 4,231,027 issued to Mann et al. A similar system is shown in U.S. Pat. No. 4,324,251 issued to Mann. From these references, it can be seen that close proximity of receiver and transmitter is anticipated. In U.S. Pat. No. 4,556,061 issued to Barreras et al., the use of either magnetic coupling or radio frequency signals is discussed. In U.S Pat. No. 5,117,825 issued to Grevious, the use of a three frequency closed loop design which renders a second order system characteristic is discussed.
From all of these references, it is clear that the current state of art generally provides for transfer between an external programming device and an implanted medical device using a radio frequency carrier employing very close spacing of the transmitting and receiving antennas. Such close spacing provides low power operation for a given minimum signal to noise ratio in accordance with the well known inverse square law.
Unfortunately, this close spacing causes the metallic case of the cardiac pacer, along with the enclosed receiving antenna, to have a major impact upon transmitter antenna tuning and loading. In practice, this means that small changes in positioning of the external antenna (the position of the implanted antenna is assumed to be fixed) can cause large percentage changes in the inter-antenna spacing. The result is that transmitter antenna loading varies greatly from patient to patient and even within a single transmission for a particular patient if the transmitter antenna is moved even slightly. Antenna tuning is similarly affected by the relative change of position of the implanted pulse generator within the transmitter field.